1. Field of the Invention
The invention pertains to apparatus for detecting the presence of objects composed, at least in part, of ferrous or magnetic materials in harvesting machines.
2. Description of the Prior Art
The subject invention is an improvement over the device disclosed in U.S. Pat. No. 3,757,501 entitled "Static Magnetic Field Metal Detector," issued to Bennett et al, and the following copending applications; Ser. No. 353,853, now U.S. Pat. No. 3,889,249 entitled Static Magnetic Field Metal Detector filed Apr. 23, 1973 in the names of Bennett et al; Ser. No. 373,565 entitled Static Magnetic Field Metal Detector, filed on June 25, 1973 in the name of William L. Garrott, and Ser. No. 529,305 entitled "Apparatus to Detect the Passage of Ferrous Material In Crop Harvesting Machines" filed Dec. 4, 1974 in the name of William L. Garrott, all of the above being assigned to the assignee of the present invention.
Generally, prior art devices, for detecting the presence of magnetic or ferrous objects, which preceded those mentioned in the foregoing paragraph, have one or more disadvantages rendering them of little value in certain applications, especially in reliable protection of expensive machinery from tramp metal, for instance. Prior art bridge devices often operate with excitation frequencies as high as 20 to 100 Kilohertz and are therefore quite susceptible to the presence of moisture, vegetation, the operator, or the like. They impose on the operator the serious burden of continuous monitoring and adjustment of the calibration of the apparatus. Even at relatively lower excitation frequencies, such detectors are adversely affected by moist conditions, thereby rendering them unsuitable for use where high accuracy, reliability and freedom from generation of false alarms is demanded.
Specifically, the apparatus of the aforementioned references, although constituting a significant improvement in the art, are nonetheless subject to some limitations. More particularly, these devices incorporate circuits to compensate for the fluctuations of the background signals which are expensive and not entirely adequate. Furthermore, these devices are disposed in locations which require the detector to supervise a relatively large scan area, reduce the sensitivity of the device and, in many cases, further require a plurality of detectors for a single basic harvesting machine.